He had not captured the supplies at HF at the time of your battle and it was not certain he would. I have no idea where the troop numbers you gave came from -- why would it have been so small? McClellan was no tiger, but he had driven the Confederates to the gates of Richmond not long ago and would not likely run from 15k Confederates.
Harpers Ferry was captured on the 15th. I’m talking about the morning of the 17th. McClellan sent the 1st Corps (9,400 men) and 12th Corps (7,600 men) to attack the confederates. Lee had four infantry divisions on that part of the field: Jackson’s division (2,100 men), Ewell’s Division (4,200 men), Hood’s division (2,300 men), and D.H. Hill’s division (5,800 men). He also had his reserve artillery (600 men). He also had some cavalry and horse artillery around Nicodemus Heights, but I didn’t count them. (Source: Ezra Carman)
McClellan wouldn’t run from 15,000 confederates, but McClellan fears that Lee has over 90,000. McClellan knows that Lee’s army was recently separated and in pieces, but Lee has had two days to reunite. McClellan thinks that he could be facing Lee’s whole army of over 90,000, or it could be less than half that if Lee’s main force is still at Harpers Ferry. Or it could be any amount in between.
So, if the 1st and 12th Corps are successful, then it tells McClellan that Lee’s force is weak, and therefore McClellan will want to throw his whole army at Lee in order to overwhelm him, which is exactly what McClellan tries to do. But if the 1st and 12th Corps are repulsed, then it will confirm to McClellan that Lee has a strong force, in a very good defensive position, and I don’t think that McClellan ever sends the 2nd, 9th, and 6th Corps across the creek, because he doesn’t want to lose his whole army. I think he sits on the eastern side of the Antietam and demands more troops.